In storage (recording) devices for information, equipment, semiconductor memory, and magnetic memory are mainly used. For short access time, semiconductor memory are used in internal storage devices and for larger capacity and non-volatility, magnetic disk devices are used in external storage devices. Storage capacity is an important index to indicate the capability of magnetic disk devices. Magnetic disk devices with huge capacity and of compact size have been increasingly requested by the market with recent developments of the information society. Perpendicular recording is a recording scheme suitable for this demand.
To promote higher recording density in the perpendicular recording, more minute recording bits are desired. Developing more minute recording bits, however, reduces the thermal stability of recording bits so that the non-volatility of magnetic information may be lost thus resulting in the memory being unsatisfactory. Accordingly, to ensure the thermal stability, recording media with higher anisotropy in the perpendicular direction are desired. It has generally been known that recording media with high anisotropy are difficult to record because of the enhanced coercivity involved.
To address this problem, a solution has been disclosed in the Published Jap. Translation of PCT International Appl. No. 2005-525663. This document describes a recording system which is comprised of a first write magnetic pole for reversing a magnetic field in accordance with writing information and a second write magnetic pole for producing a high-frequency magnetic field. This document further discloses a technique to obtain an effective magnetic reversal field which is stronger than the magnetic field from the first write magnetic poles by the synthetic magnetic field of the two magnetic poles (the second write magnetic pole assists the recording operation). In particular, a feature of this disclosure is the second write magnetic pole disposed between the first write magnetic pole and a free layer whose magnetization is controlled by spin transfer torque.
In a conventional head structure, a second write magnetic pole for generating an assisting magnetic field is affected by the magnetic field generated by a first write magnetic pole, and the oscillation of an element which is connected to the second write magnetic pole is also affected by a magnetic field from the first write magnetic pole. Hence, the oscillation frequency varies, and therefore, intended frequency control is difficult to perform.